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Lecture II
RESPIRATORY SYSTEM
The respiratory system is the biological system of any organism that engages in gas
exchange. Even trees have respiratory systems, taking in carbon dioxide and emitting
oxygen during the day, consuming carbon dioxide and producing oxygen constantly.
In humans and other mammals, the respiratory system consists of the airways, the lungs,
and the respiratory muscles that mediate the movement of air into and out of the body.
Within the alveolar system of the lungs, molecules of oxygen and carbon dioxide are
passively exchanged between the gaseous environment and the blood. Thus, the
respiratory system facilitates oxygenation of the blood with a concomitant removal of
carbon dioxide and other gaseous metabolic wastes from the circulation.
Organization
The respiratory system can be conveniently subdivided into a conducting zone and a
respiratory zone.
The conducting zone comprises:
 The nose
 The nasopharynx
 The larynx, or voicebox
 The trachea, an air tube that connects with the bronchi
 The right main bronchus and the left main bronchus tubes that carry air to and from
the lungs
 The bronchioles, branches of the bronchi which distribute air to the alveoli
 The terminal bronchioles
The respiratory zone comprises:
 The respiratory bronchioles
 The alveolar ducts
 The alveoli, terminal sacs in which gas exchange occurs
Muscles used for inspiration include:
 The diaphragm, which mediates intrathoracic pressure to initiate inspiration.
 The external intercostal muscles, during vigorous inspiration
Although expiration is generally a passive process, muscles aiding forced expiration
include:
 The abdominal muscles
 The internal intercostal muscles
Other:
The right and left bronchioles, terminal bronchioles, respiratory bronchioles, alveolar
ducts, and alveoli form the right and left lungs respectively.
The pulmonary blood vessels generally accompany the airways and also undergo
numerous branchings. The pulmonary circulation has a very low resistance compared to
the systemic circulation, and for this reason, all the pressures within the pulmonary blood
vessels are low.
Functions
The major function of the respiratory system is gas exchange. Respiration consists of a
mechanical cycle of inspiration and expiration, with gaseous exchange occurring in
between.
Inspiration is driven primarily by the diaphragm. When the diaphragm contracts, the
ribcage expands and the contents of the abdomen are moved downward. This results in a
larger thoracic volume, which in turn causes a decrease in intrathoracic pressure. As the
pressure in the chest falls, air moves into the conducting zone. Here, the air is filtered,
warmed and humidified as it flows to the lungs.
Expiration, on the other hand, is typically a passive process. The lungs have a natural
elasticity; as they recoil from the stretch of inspiration, air flows back out until the
pressures in the chest and the atmosphere reach equilibrium.
During forced inspiration, as when taking a deep breath, the external intercostal muscles
and accessory muscles further expand the thoracic cavity.
During forced expiration, as when blowing out a candle, expiratory muscles including the
abdominal muscles and internal intercostal muscles, generate abdominal and thoracic
pressure, which forces air out of the lungs.
Upon inspiration, gas exchange occurs at the alveoli, the tiny sacs which are the basic
functional component of the lungs. The alveolar walls are extremely thin (approx. 0.2
micrometers), and are permeable to gases. The alveoli are lined with pulmonary
capillaries, the walls of which are also thin enough to permit gas exchange. Oxygen
diffuses from the alveolar air to the blood in the pulmonary capillaries, as carbon dioxide
diffuses in the opposite direction, from capillary blood to alveolar air. At this point, the
pulmonary blood is oxygen-rich, and the lungs are holding carbon dioxide. Expiration
follows, thereby ridding the body of the carbon dioxide and completing the cycle of
respiration.
Oxygen Delivery System
The primary function of the respiratory system is to supply the blood with oxygen in
order for the blood to deliver oxygen to all parts of the body. The respiratory system does
this through breathing. When we breathe, we inhale oxygen and exhale carbon dioxide.
This exchange of gases is the respiratory system's means of getting oxygen to the blood.
Respiration is achieved through the mouth, nose, trachea, lungs, and diaphragm. Oxygen
enters the respiratory system through the mouth and the nose. The oxygen then passes
through the larynx (where speech sounds are produced) and the trachea which is a tube
that enters the chest cavity. In the chest cavity, the trachea splits into two smaller tubes
called the bronchi. Each bronchus then divides again forming the bronchial tubes. The
bronchial tubes lead directly into the lungs where they divide into many smaller tubes,
which connect to tiny sacs, called alveoli. The average adult's lungs contain about 600
million of these spongy, air-filled sacs that are surrounded by capillaries. The inhaled
oxygen passes into the alveoli and then diffuses through the capillaries into the arterial
blood. Meanwhile, the waste-rich blood from the veins releases its carbon dioxide into
the alveoli. The carbon dioxide follows the same path out of the lungs when you exhale.
The diaphragm's job is to help pump the carbon dioxide out of the lungs and pull the
oxygen into the lungs. The diaphragm is a sheet of muscles that lies across the bottom of
the chest cavity. As the diaphragm contracts and relaxes, breathing takes place. When the
diaphragm contracts, oxygen is pulled into the lungs. When the diaphragm relaxes,
carbon dioxide is pumped out of the lungs.
Other Important facts:
In an average resting adult, the lungs take up about 250ml of oxygen every minute while
excreting about 200ml of carbon dioxide.
The movement of gas through the larynx, pharynx and mouth allows us to speak, or
phonate.
The respiratory tract is constantly exposed to microbes due to the extensive surface area,
which is why the respiratory system includes many mechanisms to defend itself and
prevent pathogens from entering the body.
Virtually all the body's blood travels through the lungs every minute. The lungs add and
remove many chemical messengers from the blood as it flows through pulmonary
capillary bed. The fine capillaries also trap blood clots that have formed in systemic
veins.
Diseases
Diseases of the respiratory system can be classified into four general areas:
1. Obstructive Diseases (e.g., Emphysema, Bronchitis, Asthma)
2. Restrictive Diseases (e.g., Fibrosis, Sarcoidosis, Alveolar Damage, Pleural
Effusion)
3. Vascular Diseases (e.g., Pulmonary Edema, Pulmonary Embolism, Pulmonary
Hypertension)
4. Infectious, Environmental and Other Diseases (e.g., Pneumonia, Tuberculosis,
Asbestosis, Particulate Pollutants)
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